By comparing the organization, specificity and function of regulatory elements in Salmonella phage P22 and coliphage lambda, an insight has been gained into both the regulation and evolution of temperate bacterial viruses. We have undertaken a physical and functional analysis of Salmonella phage L to determine its relationship to phages P22 and lambda. We have constructed hybrid phages between P22, L and lambda in various combinations. These hybrid phages have been used to correlate genetic maps and physical maps in the region of P22 and L early genes and P22 and lambda late regulatory genes. We will continue our genetic and physical mapping to include the late gene regions (including regulatory genes) of P22 and L. This is accomplished through the DNA heteroduplex analysis of a large number of hybrid phages which have been genetically characterized and though the analysis of P22 and L themselves. We further propose to continue our investigation of P22's ability to shut off host DNA synthesis upon infection or induction. We have learned that phage L also possesses this ability. Here again, hybrid phages are very useful in mapping the location of this function on the genome and determining its properties. Lastly, we will continue our isolation and characterization of E. coli mutants which block the growth of lambda imm 22 hybrid phages. These hybrids used contain a substitution of P22 early regulation, immunity and DNA replication genes for the analogous lambda genes. E. coli mutants which block growth of lambda 22 but not lambda must be in E. coli genes whose products are required for the proper functioning of one of the P22 gene products encoded in the lambda imm 22 hybrid phage. The first mutant we have isolated blocks lambda imm 22, but not lambda imm L or lambda. It is being studied and other mutants will be isolated and studied. Phage mutants which bypass the host block will of course be isolated and characterized.